The U.S. on Wednesday sharply refuted Russian allegations that Turkey is facilitating IS oil sales, Anadolu agency reported.

“We reject the premise that the Turkish government is in league with ISIL to smuggle oil. We have seen no evidence to support such an accusation,” State Department spokeswoman Julia Mason said in a statement to Anadolu Agency.

“Turkey is taking steps to improve the security of its border with Syria, working with international partners. One goal of this effort is to cut off ISIL smuggling,” Mason added.

Following the downing of a Russian warplane that violated Turkey’s airspace near the Syrian border on Nov. 24, Russia announced sanctions against Turkey and President Vladimir Putin has alleged Turkish involvement in oil purchases from IS.

Turkish President Recep Tayyip Erdogan has challenged Putin and said he will step down if Russian calims are proved. He called on Putin to do the same if they are not.

The Russian charges were also dismissed by the White House, whose spokesman criticized Russian support for Syrian President Bashar al-Assad, whom he said is "the largest consumer of ISIL oil".

"The irony of the Russians raising this concern is that there's plenty of evidence to indicate that the largest consumer of ISIL oil is actually Bashar al-Assad and his regime, a regime that only remains in place because it is being propped up by the Russians," said Josh Earnest. "If the Russians are really concerned about ISIL's illicit finance efforts, they should take it up with Bashar al-Assad, the person that is relying on the Russians for his continued ability to remain in power."